The purpose of this study is to develop techniques for studying central nervous system monoamine metabolism in man, and to carry out pharmacokinetic studies of psychoactive drugs. Studies of the kinetics and clearance of deuterated homovanillic acid (HVA) from the circulation continued in Rhesus monkeys and patients. Problems with the application of this technique to study CNS dopamine turnover in man were resolved by further study of the kinetics and volume of distribution of the metabolite, HVA, before and after probenecid administration.